Apparatus for making geophysical explorations



March 30; 1943. EVJEN 2,314,873

APPARATUS FOR MAKING GEOPHYSICAL EXPLORATIdNS Filed Sept. 27, 1941 t 2 Sheets-Sheet 1 I mv NTogL v Ha'akon MZz/yen 1 ATTORNEY March 30, 1943. H. M. EVJEN 2,314,873

APPARATUS FOR MAKING GEOPHYSICAL EXPLQRATIONS Filed Sept. 27, 1941 2 Sheets-Sheet 2 VINVENTQR. M02221. [I v em BY v 4 ATTORNEY Patented Mar. 30, 19.43

UNITED ,STATES PATENT? orricg arrana'rusroa MAKING cnornrslcan I I 1 nxrnomrrons.

Haakon M. Evien, Houston, Tex, assignor to Nordel Corporation, Houston,

tion of Delaware v Ten, a cornora- Application September 27, 1941; Serial No. 412,622 '1 Claims. (01. 175-182) This invention relates to a method of making geological explorations and more particularly to a novel and improved method of electrical prospecting and to an apparatus for'carryingout the same.

r In the potential method of electrical prospecting, the impedance of the earth at various points and depths is determined by passing an electrical current through the earth between a pair of current electrodes and measuring the potential difference between two other points adjacent the 'earths' surface within the field of influence of the earth current thus produced. Measurements of this type require the simultaneous reading of the earth current and of the induced potential and the computation of the earths apparent re-- sistivity from these values. Such methods are diflicult to operate in the field due to the necessity for accurately regulating the current and making two accurate readings simultaneously.

Furthermore, after the readings are obtained, it is necessary to compute the corresponding apparent resistivity before the results can be interpreted.

It is an object of the present invention to provide a system of the above type in which the necessity for making a plurality of simultaneous measurements is eliminated.

Another object is to provide a measuring system which may be calibrated in units which represent the geophysical condition of the earth at the points being investigated, so that the readings may. be interpreted= directly without the necessity for mathematical calculations.

Another object is to provide a novel and improved system for measuring the geophysical aspects of the earth at various depths.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed. I v

Although the novel features which are believed to be characteristic of this invention are more particularly pointed out in the claims appended hereto, the invention itself may be better under-.

stood by referring to the following description, taken in connection with the accompanying tions illustrating different embodiments of the invention.

In accordance with the present invention, a

potential measurement is'obtained which bears a definite relationship to the current flowing through the earth-between the current electrodes. For example, the potential used; for measuring is essentially proportional to the current flowing between the current electrodes. In-

asmuch as the potential difference between the 1 two probe electrodes is also proportional to-this current, it is unnecessary to make independent measurements of either the actual current or the actual potential, because the measiu'ing potential automatically places the measured potential on a unit current basis. This furnishes a directly measured quantity immediately suitable for interpretation. I V

The present invention accordingly provides a meansfor obtaining a measuring potential which bears a definite relationship to the current flow ing in the earth circuit and may be compared with the potential induced between the measuring or'potential electrodes. 1

Ida systemof this type, a is preferable in v order to avoid current flow between the circuits,

to have no direct ohmicconnection, other than.

the earth itself, between the current portion of the circuit and the potential portion of the circuit. However, it is necessary that there be some type of coupling between the source of the measuring potential and the current branch of the circuit in order to obtain a definite relation between the current and the measuring potential.

In accordance with the present invention, this is accomplished by alternately charging a condenser to a potential determined by the potential drop across a resistance connected in the earth current circuit; and then discharging said Candenser into a second condenser connected across a potentiometer. The potential drop thus developed across the potentiometer bears adefinite relationship to the current flowing in the first mentioned resistance and in the earth circuit.

drawings, in which certain specific embodiments thereof have been set forth for purposes of iilustration.

In the drawings, a a

Fig. l is a schematic representation of an electrical prospecting system embodying the present invention; and

Figs. 2 and 3 are similar schematic representa- The measuring potential is taken from the potentiometer and may be adjusted to exactly counter-balance the potential being measured. A galvanometer or the like may be used for determining when an exact balance is reached.

The only reading which is required in this system is thesettlng of the potentiometer by which the measuring potential is obtained; This potentiometer may be calibrated in sultableunits which correspond to the earth's electrical characteristics and may be interpreted to indicate tery.

' .to twenty cycles per second, are necessary in order to penetrate to substantial depths such, for example, as depths below. 2000 ft. High frequency currents do not penetrate the earth to any great distance. The present system accordingly is adapted for obtaining a set of measurements, termeda potential spectrum, taken between fixed points at different low frequencies. A set of such potential spectra taken at different locations on the earth's surface may be interpreted according towell known electrical prospecting methods to give an indication of the nature and distribution of the subterranean strata. Such measurements are particularly useful in prospecting for oil bearing strata and the presence of oil may frequently be detected directly due to .the electrical characteristics of the oilbearing strata as compared to the usual earth strata. v

The extremely low frequenciesjmay be readily obtained by various means, such, for example, as

a commutator driven at controlled speeds, or by potential is obtained for purposes of measurement.

One embodiment of this invention is shown more in detail in Fig. .1. Referring to this flgure, the earth current circuit comprises a'source "I of direct current, such as a generator or batwith a line H, a. resistance l2, an ammeter l3 and a line l4.. Lines II and H are connected to segments 15 and I8 respectively of a commutator The source I0 is'conneeted in series circuit I! which is .driven' by suitable means to be de- 1 tance apart, which may be of. the order of several-thousand feet, and when the commutator I1 is drivenv at a suitable speed, an alternating current having a frequency corresponding to the speed. of rotation of the commutator, passes through the earth circuit. This current may be measured by the ammeter l3 and may be adjusted within desired limits by source l0. It will be noted that a potential drop is built suitable regulation of the potential drop across saidresistance.

strips 28 and 83 are offset so that the rotary switches are never closed at the same time. The condenser 30 is connected with resistances 38 and leads 38a. across a potentiometer 39 having variable contacts 40 and 4|, vThe contact 40 is connected by a line 42 to a galvanometer 43 and thence through a line 44 to a reversing commutator 48. The contact 4| is connected by a. line 41 to said commutator 46. Segment 45 of the commutator 46 is connected by a line 50 to an earth potential electrode 5|: Segment 48 of the commutator, is connected by a line 52 to the movable contact 53 of a potentiometer. 54 which is connected across a battery 55. The'midpoint of this battery 55 is connected by a line 56 to an earth potential electrode 51. The commutators l1 and 46 are shown as mounted on a common shaft 60 and as driven in synchronism by a motor 8| through a suitable control box 82 which may comprise a gear box and a rheostat for adjusting the motor speed. A control handle 63 is provided for controlling the motor speed as desired. The speed of rotation of the commutator may be indicated by a speed indicator G4.

The galvanometer 43 is preferably of the highly damped type so that it is influenced only by the direct current component and not by any ripple or other alternating current component in the circuit under measurement.

Operation In the operation of this system, the current electrodes 20 and 2| are located at selected points in the earth and the potential electrodes 5| and ll remains stationary. For low frequency alternating current, the commutator is slowly rotated at the selected speeds as indicated by the speed indicator 64. It will be noted that the potential drop in the resistance l'|- will vary in proportion to the amount of current flowing in the earth circuit.

* The condenser 25 which is intermittentlyconnected across'the resistance I2 by means of the rotary switch 29' receives ateach such connection a charge which is proportional to the The circuit between the condenser 25v and the resistance I2 is then broken by the switch 28 and the circuit between the condenser 25 and the conup across the resistance l2 which is directly propontional to the current flowing in the earth circuit. This potential drop is utilized in the present system for obtaining the measuring. poteni denser 25 is intermittently connected across a second condenser 30 by pairs of contacts 3| and 32 which are intermittently closed by contact strips .33 of a=rotary switch 34. The rotary switches 29 and 34 are shown as mounted on'a shaft 35 and driven by-a motor 36. The contact denser 30 is established by the closing of the switch. 34. The condenser 25 then discharges intothe condenser 30, which accumulates the successive charges and attains a potential which is proportional to the average potential drop across the resistance l2 and which varies in ac- 'cordance with fluctuations in said potential maintained at an average value proportional to the potential drop across resistance l2.

The charge upon the condenser 30 deter min es the voltage drop across the potentiometer 39 .and the average value of this voltage drop is If the measurements are to be taken with direct current, the commutator ,at all times proportional to the voltage drop through the resistance 12 which, in turn, is

determined bythe total current flowing in the earth circuit.

With the circuit operating in this manner, suitable adjustments are made 01' the contacts 4. and 4| to produce a zero reading on the galvanometer '43 which indicates that the average voltage drop across the intermediate portion of the potentiometer 39 between the contacts 44 and 4| exactly balances the potential which is picked up by the earth potential electrodes II and I1 after the latter has been rectified by commutator 46. v

The potential picked up by the potential electrodes corresponds in frequency to the frequency of the earth current. The alternating potential is rectified by the commutator 48 which operates in synchronism with earth current commutator I! to supply a direct voltage across the galvanometer 43' and to the potentiometer 39. I

The eiifect oiany residual earth current which would tend to produce a constant direct, potential across the electrodes BI and 51 is elimipendent upon the total value oi the earth current at the instant that the measurement is being taken, automatically compensates for any 7' variations in electrode resistance or in contact resistance andeliminates any necessity for making simultaneous measurements oi this earth current and the induced potential.

This measurement is repeated with different frequencies until a complete potential spectrum of the area between electrodes ii and 51 is obtained. These electrodes may then be moved to the' depth which is under investigation at any nated by suitable adjustment of the potentiometer 54. This adjustment may be made, for example, while no current is flowing through the earth circuit so that the only eflect on the.

galvanometer 43 represents the eflect of the residual earth current. Since the earth current impresses a direct current potential across the probe (or potential) electrode. any unbalancefbetween this potential and the potential taken from potentiometer 54, will appear as an alternating potential after commutation by commutator 46. This will make the galvanometer kick from one side to the other in synchronism with the commutation. Any drift in the natural earth potential may thus be immediately perceived, and can be compensated by readjusting potentiometer 54 until the kicking of the galvanometer is eliminated.

The potentiometer .39 is never connected directly .to the resistance l2 inalsmuch as the switches 28 and 34 are never closed at the same time. Consequently, there is no direct ohmic connection in thecoupling between potentiometer 39 and resistance l2. 'Even though the lowest potential of resistance l2 may be considerably higher than ground potential, and potentiometer 39 is directly connected to ground by leads s1 and 50, it is impossible for the potential difierence between resistance l2 and electrode 5| to produce a current. Thus, only the diflerential drop of potential across resistance I! is efiective ,in producing the potential across potentiometer 39, and this potential'may be used forthe purpose of measurement without further complications. A filter may be included in the measuring circuit if desired in order to eliminate ripples or fluctuations from various causes such, for example, as those proparticular instant.

It will be noted that in the above described system only a single reading need be taken and the necessity for mathematical calculations is avoided. The system is accordingly well adapted to field use and may be operated by unskilled persons. The data thus compiled may be interpreted by geological physicists in accordance with well known principles. i I

The size of the electrodes may vary over wide limits. In practice; metals'takes about one or two inches in diameter'and 3 ft. or 4 ft. inlength have been found satisfactory. The'distance between the current electrodes may vary widely, for example, from 200 ft. to 5000 ft., depending upon the depth to be investigated. The potentialof the source 10 may likewise be adjusted as desired. Voltages of 100 volt to 200 volts and earth currents of one to two a peres have been found'satisfactory.

In the embodiment shown in Fig. 2 parts corresponding to Fig. lare given the same reference numbers. The coupling circuit indicated by the from the direct current side of the current commutator IT as in Fig. 1, but is applied to the alternating current side of the commutator 46.

. The segment of the commutator 46 is cona definite earth characteristic at adepth which is determined by the penetration of the earth current at the particular frequency at .which the mesurement is being taken. The potential [12 and 13 respectively of a commutator 14 which is mounted on the shaft and is driven in SW- chronism with the commutators l1 and 4B. The

commutator I4 isconnected by lines 15 and 16 across a potentiometer 18 having variable con-. tacts l9 and-80.. The contact 19' is connected by a line 82 to the segment 45 of the commutator 48. The contact is connected by a line 8| to the potential electrode 51. The remaining elements are identical with those shown and described in Fig. l.

In this embodiment the .measuring potential derived from the condenser 30 is reversed by means of commutator 14 in synchronism with the reversals of the earth current and is applied to the potential circuit on the alternating current side of the commutator 48. The potential dedeveloped across the potentiometer 39 being deti rived from the potential electrodes 5| and '51 is balanced against the potential taken from the potentiometer 18, is rectified by the commutator 46 and is applied as a direct current to the galvanometer 43. The operation is otherwise identical with the system shown in Fig. 1.

In the embodiment shown in Fig. 3 the various parts are similar to Fig. l and given corresponding reference characters. In this embodiment, however, the coupling circuit indicated by the rectangle 24 is connected between the alternating current side of the commutators I! and 46. In this embodiment the ammeter I3 is connected by the lead I I directly to segment l5 of the commutator H. The resistance I2 is connected in the lead IS. The potentiometer l8 is connected as shown in Fig. 2 to the leads 8| and 82 between the-commutator 46 and the potential electrode 5|. The other elements are connected as previously described and operate in; a similar manner.

It is to be understood that the commutators I7, 45 and 14 are shown as rotary commutators merely for purposes of illustration and that any suitable synchronized reversing means may be used for reversing the current in the respective circuits so as to obtain the balancing and measuring eflect above described. Furthermore, any

, suitable means 'may be used for controlling the rate of operation of such commutators for thereby varying the frequency of the measuring current'so as to obtain the desired frequency specand across said first condenser, whereby said sec- 0nd condenser periodically receives a charge from the potential drop across said resistance and applies the same to said first condenser for thereby building up a charge in said first condenser which is proportional to the potential drop across said resistance.

trum of the various localities under'measure-f ment. likewise, the switches 29 and 34 may be of any desired form suited to alternately connect the condenser 25 to the resistance I2 and to the condenser 30 and to avoid the establishment of adirect ohmic connection at any time between the condenser 30 and theearth current circuit.

Although certain specific embodiments of the invention have-been shown for purposes of i1lustration, it is to be understood that the invention is not to be limited thereto but that various changes andmodifications may be made therein tween said electrodes, a potential circuit includ-- ing potential pick-up electrodes located at spaced points adjacent the earth's surface lying within 2. A system for electrical prospecting, comprising an earth current circuit including a source of direct current, a pair of spaced earth electrodes, and reversing means connected between said source and said electrodes for reversing the direct current at a predetermined controlledfrequency whereby an earth current having. a predetermined frequency ispassed through the earth between said electrodes, 8. potential circuit including potential pick-up electrodes, a potential circuit including potential pick-up electrodes located at spaced points adjacent the earths surface lying within the field of influence of said earth current, a direct current indicating device connected to indicate the potential of said potential circuit, and reversing means synchronized withsaid first reversing means and connected between said potential pick-up electrodes and said indieating device to convert the picked-up potential difference into a uni-directional potential suited sistance connected in series with said earth curas will readily appear to a person skilled in the the field of influence of said earth current, a di 1 rect current indicating device connected to indicate the potential of-said potential circuit, and reversing means synchronized with said first reversing means and connected between said potential pick-up electrodes andsaid indicating device 1 current'flowing in said earth current circuit, a-

rent circuit to develop a potential drop thereacross proportional-to the. current flowing in saidearth current circuit, a potentiometer connected in said potential circuit to apply thereto a poten-.

denser across said resistance and across said first condenser, whereby said second condenser periodically receives a charge from the potential drop across said resistance and applies the same to said first condenser for thereby building up a charge in said first condenser which is proportional to the potential drop across said resistance, and high resistances connected in series circuit with said first condenser and said potentiometer to retard the'leakage of thecharge from said -flrst condenser between successive applications of charges thereto fromsaid second condenser wherebysaid first condenser retains a charge which is proportional to the across' said-resistance. v

3. A syst m for electrical prospecting, comprising an earth current circuit including a source of direct current, a pair of spaced earth electrodes, and reversing means connected between said I potential drop source'and-said. electrodes for reversing the direct currentat a predetermined controlled fre;

quency whereby an earth current having a pres determined frequency is passed through the earth between said electrodes, a potential circuit in eluding potential pick-up electrodes located at spaced points adjacent the earth's surface lying within the field of influence of said earth current, a direct current indicating device connected to indicate the potential of said potential circuit, and reversing means synchronized with said first reversing means and connected between said earth current circuit, a potentiometer connected in said potential circuit to apply thereto a po- 6. The system set forth in claim 1 in which said resistance is connected on the .direct current side of the reversing means of said earth current circuit, the potentiometer is connected on the alternating current side of the reversin means of said potential circuit, and a reversing means synchronized with said first reversing means is provided for reversing the coupling to said potentiometer from said coupling circuit,

whereby the potential drop in said potentiometer corresponds in direction to the potential pick-up by said potential pick-up electrodes.

'7. A system for electrical prospecting, comprising an earth current circuit including a pair of spaced earth electrodes, a source of current connected thereto, reversing. means connected bee tween said source and said electrodes for reversing the earth current'at a predetermined con-- tential drop opposing the potential picked up from said potential pick-up electrodes, a condenser connected across said potentiometer, a second condenser, and synchronized switch means connected to alternately connect said second condenser across said resistance and across said first condenser whereby said second condenser periodically receives a charge from the potential drop across said resistance and applies the same to said first condenser for thereby building up a charge in said first condenser which is propor tional to the potential drop across said resistance, and means in said potential circuit to balance the residual earth potential, said means comprising a potentiometer connected in said earthcurrent circuit and a source of potential connected across.

said potentiometer to produce a voltage drop therein, said potentiometer having adjustable contacts whereby'the value of said balancing potential may be adjusted as required.

4. The system set forth in claim 1 in which said resistance is connected on the direct cur, rent side of the reversing means of said earth current circuit and said otentiometer is connected on the direct current side of the reversing means of said potential circuit.

5. The system set forth in claim 1 in which said resistance is connected on the alternating current side of the reversing means of said earth current circuit and said potentiometer is connected on the alternating current side of the reversing means of said potential circuit.

trolled frequency, a potential circuit including potential pick-up electrodes located at spaced points adjacent the earth's surface lying within the field or influence of said earth current, a direct current indicating device connected to indicate the potential of said potential circuit, reversing means synchronized with said first reversing mean and connected between said 130-,

tential pick-up electrodes and said indicatin device to convert the picked-up potential difference into a uni-directional potential suited to actuate said indicating device, and coupling means connected between said earth current circuit and said potential circuit to impress on said potential circuit a balancing potential which is directly proportional to the current flowing in said earth current circuit at the instant of measurement, said coupling means comprising a resistance connected with said earth current circult to develop a potential drop thereacross proportional to the current flowing in said earth current circuit, a condenser, switch means operating successively to connect said condenser across said resistance to be charged in accordance with the voltage-drop thereacross and to disconnect said condenser from said resistance and to connect said condenser to said potential circuit to supply thereto a potential opposing the picked up potential, and means adjusting the relationship between the potential drop across said resistance and the. potential supplied to said pick-up circuit by said' condenser-to obtain a balancing potential suited to produce a zero reading in said indicating device.

HaAKoN M. EVJEN. 

